Power steering system with auxiliary pump

ABSTRACT

Described is a vehicle power steering system having a primary pump and a secondary pump for supplying hydraulic fluid to a steering assist mechanism. Sensing means, responsive to fluid mass flow rate through an orifice, are provided for causing actuation of valve means to disconnect the primary pump, upon failure thereof, from the steering assist mechanism and to connect the auxiliary pump thereto.

United States Patent [1 1 Feustel et al.

1451 July 24, 1973 POWER STEERING SYSTEM WITH AUXILIARY PUMP [75]Inventors: James R. Feus'tel, Taylor; Warren A. Van Wicklin, Jr.,Dearborn, both of Mich.

[73] Assignee: Ford Motor Company, Dearborn, Mich. [22] Filed: Mar. 15,1971 211 Appl. No.: 124,081

[52] US. Cl. 180/792 R, 60/405 [51] Int. Cl 862d 5/08 [58] Field ofSearch. ISO/79.2 R; 60/52 S,

[56] References Cited UNITED STATES PATENTS Schubert et a] 180/792 R2,954,671 10/1960 Kress 60/525 3,407,894 10/1968 Thompson et al. 180/792RX Primary Examiner-Benjamin Hersh Assistant Examiner-John A. PekarAttorney-John R. Faulkner and Robert W. Brown [57] ABSTRACT Described isa vehicle power steering system having a primary pump and a secondarypump for supplying hydraulic fluid to a steering assist mechanism.Sensing means, responsive to fluid mass flow rate through an orifice,are provided for causing actuation of valve means to disconnect theprimary pump, upon failure thereof, from the steering assist mechanismand to connect the auxiliary pump thereto.

11 Claims, 3 Drawing Figures Patented July 24, 1973 3,747,725

2 Sheets-Shoot 1:

James A? feas/e/ ATTORNEYS POWER STEERING SYSTEM WITH AUXILIARY PUMPBACKGROUND OF THE INVENTION This invention relates to a vehiclehydraulic power steering system having a primary pump'and a secondary orauxiliary pump. The invention is particularly suitable for use in avehicle power steering system of this kind wherein the primary pump isdriven by the vehicle engine and normally supplies hydraulic fluid to asteering assist mechanism and wherein the auxiliary pump is drivendirectly or indirectly by the vehicle wheels and is automaticallyconnected to the steering assist mechanism upon failure of the primarypump or the hydraulic circuitry connected thereto.

Power steering systems of the kind described above are well known. Anexample of such a system may be seen in US. Pat. No. 3,407,894, issuedOct. 29, 1968, in the name of William B. Thompson et al.

A suitable secondary power steering system should furnish sufficienthydraulic fluid flow at adequate pressure to provide normal powersteering performance at vehicle forward speeds in excess of apredetermined low value, such as 5 miles per hour. Moreover, thesecondary system should be responsive to any significant reduction inhydraulic fluid flow rate to the steering assist mechanism from theengine driven pump to quickly actuate the means for connecting theauxiliary pump to the steering assist mechanism. Also, the sensingmechanism used to indicate a primary pump or hydraulic circuit failureshould function over the entire steering system operating temperaturerange and should be able to distinguish between the flow of hydraulicfluid having little or no air content and hydraulic fluid having asubstantial quantity of air mixed therewith. Furthermore, the secondarysystem should operate quietly, have a durability equivalent to that ofthe primary power steering system, operate independently of the engineand electrical system, and have a single hydraulic fill point where thehydraulic fluid level for both the primary and auxiliary pump andhydraulic circuits may be checked.'The power steering system of theinvention can satisfy these requirements.

SUMMARY OF THE INVENTION In order for a secondary power steering systemto be operable, means must be provided for sensing a failure of theprimary power steering system or pump to cause the secondary system tocome into operation. It has been proposed by others that this sensingmeans comprise a pressure switch which would energize a secondary orauxiliary pump, or that a valve be used which senses the primary pumppressure and shifts upon loss thereof. However, these methods'fordetecting a failure of the primary power steering supply system are notentirely satisfactory because failure of the primary hydraulic powersteering supply system is determined by sensing the pressure of theprimary hydraulic supply circuit and comparing that pressure with somemore or less arbitrary reference pressure. In most vehicle powersteer-ing systems in use today, an open-center valve is used in thesteering assist mechanism so that the primary pump output pressure isquite low except during steering maneuvers. In the event of a primarysystem failure where there is a total or partial loss of hydraulicfluid, the primarypump acts as an air compressor and may supply anoutput pressure substantially equivalent to the low hydraulic fluidpressure which occurs during normal primary system operation at timesother than during a steering maneuver. Thus, the prior art pressuresensing means may not detect all cases of failure in the primaryhydraulic supply circuit.

In accordance with the invention, a vehicle power steering system havinga primary and a secondary or an auxiliary pump includes a transfer valvefor altering the fluid flow circuits of the primary and auxiliary pumps.Operation of the transfer'valve is controlled by pressure differentialsensing means which determine the mass flow rate in the primaryhydraulic supply circuit. Should the mass flow rate become insufficientin magnitude, the transfer valve disconnects the primary hydraulicsupply circuit from the steering assist mechanism and connects theauxiliary hydraulic supply circuit to the steering assist mechanism sothat powerassisted steering is retained.

The rhass flow rate which controls the operation of the transfer valveis sensed by causing the fluid in the primary hydraulic supply circuitto flow through fluid flow restrictive means preferably i'nthe form of acalibrated orifice. This produces a pressure differential across theorifice which may be used to control the operation of the transfervalve. The magnitude of the pressure differential is determined in partby the density characteristics of the fluid passing therethrough. Thus,if the hydraulic fluid becomes mixed with air, for example, by a failurein the primary circuit, the pressure differential across the orifice isreduced and this reduction is used to cause the transfer valve toshuttle, thereby, to connect the auxiliary pump to the steering assistmechanism. A one-way valve, such as a check valve, is placedhydraulically in series with the orifice and the pressure differentialmay be taken across this combination of means for restricting fluidflow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevational viewof components which may be utilized in the practice of the invention;

FIG. 2 is a schematic diagram of hydraulic circuitry arranged inaccordance with the invention; and

FIG. 3 is a detailed sectional view of a steering gear havingincorporated therein and integral therewith a transfer valve and apressure sensing means constructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION With reference now to FIG. 1,there is shown a schematic elevational view of a motor vehicle powersteering system having components which may be utilized in practicingthe invention. The system includes a primary hydraulic circuit the mainelement of which is a hydraulic pump 10, preferably of the positivedisplacement type, having as a means for driving it a pulley 12. Thepulley 12 of the primary hydraulic pump 10 is driven by a belt 14. Avehicle engine 16 has an output shaft and pulley 18 to which-thebeltpulley 14 is connected to rotate the drive pulley 12 of the primaryhydraulic pump 10. The primary hydraulic pump 10 has an output line 20and a return line 22.

A steering assist mechanism in the form of a power steering gear unit 24preferably is connected to the vehicle chassis in a known manner. Thepower steering gear unit 24 preferably is of the type having a pistonwith an integral worm gear which is moveable upon the application in aknown manner of fluid pressure to one side or the other of it. The powersteering gear unit includes a pitman arm 26 which is attached tosteering linkage (not shown) and which rotates through an arc to causemovement of the vehicle wheels.

A combination fluid mass flow rate sensing means and transfer valve 28is attached to, and preferably is integral with, the steering gear 24.

The backup power steering system includes a secondary or auxiliaryhydraulic pump 30. For convenience of illustration, the auxiliary pump30 is shown as being driven by a belt and pulley combination 32 attachedto the output shaft 34 of a transmission 36 connected to the engine 16.The transmission output shaft 34 conventionally is connected to avehicle drive shaft 38 which, in turn, is connected to the drive wheelsof the motor vehicle. Rotation of the drive wheels (not shown) causesrotation of the belt-pulley assembly 32 and of the auxiliary pump 30.Thus, auxiliary pump 30 is driven, directly or indirectly, by the drivewheels of the vehicle, while the primary hydraulic pump is driven by thevehicle engine. The auxiliary pump 30 could be mounted within thehousing of the transmission 36 and might be similar in design to thehydraulic pump described in the aforementioned U.S. Pat. No. 3,407,894.Also, the auxiliary pump 30 could be driven by the rear axle ordifferential of a vehicle so equipped.

The auxiliary pump 30 has a fluid output line 40 and a fluid return line42. These output and return lines 40 and 42 are connected to thecombination sensing means and transfer valve 28, as are the output andreturn lines and 22 of the primary hydraulic pump 10. Preferably, boththe primary hydraulic pump 10 and the auxiliary hydraulic pump 30 haveintegral reservoirs to which the respective pump return lines 22 and 42are connected. The primary and auxiliary hydraulic pumps may have acommon reservoir, separate reservoirs, or physically separate reservoirsinterconnected by a hydraulic line 44. It is preferred that thereservoirs be interconnected so that they may have a common fluid-filllocation, which may also serve for checking hydraulic fluid level in therespective pumps.

With reference now to FIG. 2, there is shown a schematic diagram ofhydraulic circuitry constructed in accordance with the invention. Thehydraulic circuitry includes the engine driven pump 10 having areservoir 46. The pump 10 is enclosed by dotted lines to illustrate thatit is preferably located in the engine area of the motor vehicle. Thesecondary or axuiliary hydraulic pump 30 is also enclosed by dottedlines to illustrate that it is preferably in the drive line area, and itincludes a reservoir 48. The reservoirs 46 and 48 are preferablyinterconnected by the hydraulic line 44 to permit fluid flow in eitherdirection as shown in FIG. 2.

The combination massflow rate sensing means and transfer valve 28includes a pilot-operated, five-way, two-position transfer valve 50. Thetransfer valve 50 is shown as having three positions 52, 54, and 56.However, only positions 52 and 56 are stable positions. Position 54 isan intermediate unstable valve position which occurs as the valve shiftsfrom one of its stable positions to the other.

The mass flow rate sensing means comprises an orifice 58 and one-wayfluid flow means in the form of a check valve 60, the orifice 58 and thecheck valve 60 being hydraulically connected in series with one another.Preferably, the steering gear 24 and the remainder of the items enclosedwithin the dot-dash lines of FIG. 2 are contained within an integralpower steering gear assembly.

In operation, the fluid output from the pump 10 passes through itsoutput line 20, through the orifice 58 and check valve 60 where apressure drop occurs, through a passage or hydraulic line 62, and intothe steering gear 24. The orifice 58 may be calibrated or sized, forexample, such that at normal flow rates a pressure differential of about15 pounds per square inch is produced across the orifice. The checkvalve 60, the primary function of which is to prevent reverse flow fromthe power steering gear assembly into the output line 20, preferably hasa cracking pressure of approximately 2 pounds per square inch. Thecombined pressure drop across the orifice 58 and the check valve 60provides an indication of the mass flow rate therethrough. The pressureat the upstream side of the orifice 58 is applied through the hydraulicline or passage 64 to a pilot area 66 of the transfer valve 50. Thus,the pressure in the pump output passage 20 is applied through thehydraulic line or passage 64 to the pilot sensing area 66 to urge thevalve 50 toward its stable position 56. When the transfer valve is heldin this stable position 56, the fluid from the steering gear 24 passesthrough hydraulic line or passage 68 and the transfer valve and isreturned by return line 22 to the engine-driven pump reservoir 46. Thepressure on the downstream side of the check valve 60 is applied throughthe hydraulic lines or passages 70 and 72 to another pilot area 74 ofthe transfer valve 50. The fluid pressure in this pilot area 74 combinedwith the force produced by resilient means in the form of a spring 76urge the transfer valve toward its other stable position 52. However, ifthe pressure differential across the orifice 58 and check valve 60 issufficiently great, the fluid pressure in the pilot area 74 of thetransfer valve is insufficient to cause the valve to change from itsstable position 56 to its other stable position 52.

When the transfer valve 50 is in its stable position 56, the output fromthe auxiliary pump 30 enters the transfer valve from output line 40 andis returned directly to the reservoir 48 through the return line 42.However, should the mass flow rate through the orifice 58 and the checkvalve 60 decrease below a predetermined level, the transfer valve 50shifts to its intermediate, unstable position 54 wherein the output fromthe steering gear through line 68 is connected to both of the pumpreservoirs 46 and 48 and then is shifted to its stable position 52.

When the transfer valve 50 is held in its stable position 52, the outputfrom the auxiliary pump 30 passes through output line 40, through thetransfer valve, and into the hydraulic line or passage 78. Fluidpressure is applied at this point through the passage 72 to the pilotarea 74 of the transfer valve which tends to maintain the valve in itsstable position 52. Also, fluid flows into the passage 70 to the inputline 62, and from there, into the steering gear 24. The fluid flowingout of the steering gear enters line 68, passes once again through thetransfer valve 50, and is returned by return line 42 to the auxiliarypump reservoir 48. At this time, the pressure in the line 62 is greaterthan the pressure in the line 20 so that flow through the check valve 60and orifice 58 is prevented. Because the auxiliary pump 30 is operatedby the vehicle drive wheels rather than by the engine, it will continueto produce an output as long as the drive wheels rotate. However, inpractice, a usable output from this pump is and need only be produceduntil the vehicle speed has decreased to a low predetermined value, suchas within the range from 2 to 5 miles per hour.

FIG. 3 is a detailed sectional view of the power steering gear assemblyillustrated schematically in .FIG. 2. FIG. 3 shows a portion of thetypical steering assist mechanism 24 having an input shaft 80, whichnormally is connected with the vehicles steering wheel. Shown mountedintegrally with the steering assist mechanism 24 is the combination massflow rate sensing means and transfer valve, these being designatedgenerally by the numeral 28. Specifically, the mass flow rate sensingmeans is comprised of the orifice 58 and the check valve 60. Thetransfer valve is specifically denoted by the numeral 50.

The steering assist mechanism or steering gear 24 conventionally has anopen-center valve spool 82 which moves to the right or to the leftasviewed in FIG. 3 in accordance with rotation of the steering gear inputshaft 80. Movement of the steering gear valve spool 82 in eitherdirection causes an increase in hydraulic fluid pressure applied to thesteering mechanism through the hydraulic input passage 62. Hydraulicfluid flows out of the steering assist mechanism 24 through the outputline formed by the chamber 84 and the passage 68 connected thereto.

The transfer valve 50 includes a valve body 86 having a cylindricalopening 88 therein in which axially movable spool 90 is located. Thevalve spool 90 has equal diameter lands 92, 94, 96, and 98. The valvebody 86 has ports 100 and 102. The width of the land 94 must be lessthan the width of the port 100 in order for the transfer valve 50 toprovide an unrestricted passage for the hydraulic fluid flowing from thesteering gear 24 through the passage 68 for all valve spool positions.This insures the availability at all times of an unrestricted passagefor hydraulic fluid displaced by the valve spool 90 from the pilot areas66 and 74 to allow rapid operation of the transfer valve throughintermediate position 54 shown in FIG. 2. The width of the land 96should be equal to or greater than the width of the port 102. Theprojection 104 at the left end of the valve spool 90 limits its travelto the left as viewed in FIG. 3. Spring 76 located in the pilot area 74of the transfer valve 50 urges the valve spool 90 toward the left. Whenthere is fluid input pressure through the hydraulic input line 20 fromthe engine driven primary pump 10 through the passage 64 into the pilotarea 66 at the left end of the valve spool 90, the valve spool is urgedtoward the right against the combined force of the bias spring 76 withthe force produced by fluid pressure in pilot area 74. Travel of thevalve spool toward the right is limited by the projection 106. Theeffective areas of the valve spool 90 in pilot areas 66 and 74 should beequal.

FIG. 3 illustrates the position of the valve spool 90 when neither theengine driven primary pump 10 nor the auxiliary pump 30 are producing afluid pressure as would bethe situation with the vehicle standing stilland the engine at rest. This valve position is also the position itassumes upon failure of the primary hydraulic supply system and itcorresponds to the stable position 52 illustrated schematically in FIG.2.

If the vehicle engine is running and the engine driven hydraulic pump 10is producing a normal fluid output, then the pressure acting on thepilot area 66 of the valve spool 90 is sufficient to move it toward theright against the force of the bias spring 76 combined with the forceproduced by the fluid pressure in passage acting through the passsage 72on the area of the spool located in the pilot area 74. This condition ismaintained only as long as the fluid mass flow rate through the orifice58 and the series connected check valve 60 is sufficient to produce apredetermined pressure drop between the hydraulic input line 20 and thepassage 70 so that the force acting in the pilot area 66 on the valvespool urging it toward the right is greater than the combined forcesproduced by the bias spring 76 and the fluid in the pilot chamber 74urging the valve spool toward the left.

As an example of the operation of the transfer valve and sensing meansin the event of a failure in the primary hydraulic supply circuit, itmay be assumed that a leak develops in the primary return line 22. Inthat event, the hydraulic fluid level in the primary pump reservoir 46gradually decreases as the pump continues to deliver fluid to thesteering assist mechanism 24. The pump will begin to deliver anair-hydraulic fluid mixture. Because of the reduced density of thisairhydraulic fluid mixture, the pressure drop across the orifice, 58 andthe check valve 60 decreases. If the operation of the transfer valve 50were controlled only by the output pressure of the primary hydraulicpump 10, the failure could go undetected because of the primaryhydraulic pump 10 acting as an air compressor and maintaining the outputpressure substantially constant. However, with the apparatus of theinvention, the mass flow rate is sensed and this decreases when thedensity of the hydraulic fluid decreases so that the valve spool 90moves through its unstable intermediate position 54 (FIG. 2) into itsstable position 52, the position of the valve spool illustrated in FIG.3, wherein hydraulic fluid is supplied by'the auxiliary pump 30. Thisresult is achieved because the reduced mass flow rate through theorifice produces a reduction in the fluid pressure acting through thepassages 70 and 72 on the portion of the valve spool 90 located in thepilot area 74 of the transfer valve.

Based upon the foregoing description of the invention, what is claimedand desired to be protected by Letters Patent is:

l. A vehicle power steering system which comprises: a primary pumphaving an output line and a return line; a secondary pump having anoutput line and a return line; means connected with said primary pumpoutput line for sensing mass flow rate of fluid therein; and valve meansconnected to, and for controlling fluid flow in, said primary andsecondary output and return lines, said valve means being connected tosaid sensing means and being responsive at a predetermined mass flowrate, sensed thereby to alter the fluid flow circuits of said primaryand secondary pumps.

2. A vehicle power steering system in accordance with claim 1, whereinsaid means for sensing mass flow rate of fluid comprises an orificeconnected with said primary pump output line.

3. A vehicle power steering system in accordance with claim 2, whereinsaid means for sensing mass flow rate of fluid further includes one-wayvalve means connected in series with said orifice.

4. A vehicle power steering system, which comprises: a primary pumphaving an output line and a return line;

a secondary pump having an output line and a return line; meansconnected with said primary pump output line for sensing mass flow rateof fluid therein; a steering assist mechanism; and transfer valve meansconnected to, and for controlling fluid flow in, said primary andsecondary output and return lines, said valve means having a firstposition in which said primary pump output and return lines areconnected to said steering assist mechanism and wherein said secondarypump output and return lines are connected together by said valve means,and said valve means having a second position wherein said secondarypump output and return lines are connected to said steering assistmechanism for fluid flow therethrough.

5. A vehicle power steering system in accordance with claim 4, whereinsaid means for sensing mass flow rate of fluid comprises fluid flowrestrictive means for producing a fluid pressure differential.

6. A vehicle power steering system in accordance with claim 4, whereinsaid means for sensing mass flow rate of fluid comprises an orificeconnected in series with one-way valve means to produce a pressuredifferential in the fluid output from said primary pump when said valvemeans are in said first position.

7. A vehicle power steering system in accordance with claim 6, whereinsaid transfer valve means further include means responsive to areduction below a predetermined level of the pressure differentialacross said orifice and one-way valve means to shift said transfer valvemeans from its first position to its second position.

8. A vehicle power steering system in accordance with claim 7, whereinsaid transfer valve means has a third unstable intermediate positionwherein the output from said steering assist mechanism is connected tosaid return lines of said primary and secondary pumps.

9. A vehicle hydraulic power steering system, which comprises: a primaryhydraulic fluid pump having an output line and a return line; anauxiliary hydraulic fluid pump having an output line and a return line;a fluid-controlled open-center-valve steering assist mechanism havinginput and output lines connected thereto; a transfer valve connected tothe input and output lines of said steering assist mechanism, to theouput line of said auxiliary pump, and to the return lines of saidprimary and auxiliary pumps, said transfer valve having a first positionwherein said output line of said steering assist mechanism is connectedto said return line of said primary pump and wherein said output line ofsaid auxiliary pump is connected to said return line of said auxiliarypump, and said transfer valve having a second position wherein saidoutput line of said auxiliary pump is connected to said input line ofsaid steering assist mechanism and wherein said output line of saidsteering assist mechanism is connected to said return line of saidauxiliary pump, said transfer valve further including first pilot meansconnected to said output line of said primary pump for urging saidtransfer valve to its first position and second pilot means connected tosaid input line of said steering assist mechanism for urging saidtransfer valve to its second position; and orifice and one-way valvemeans for sensing mass flow rate of fluid flowing therethrough, saidorifice and one-way valve means being connected between said output lineof said primary pump and said input line of said steering assistmechanism.

10. A vehicle hydraulic power steering system in accordance with claim9, wherein said transfer valve has a third unstable intermediateposition wherein said output line of said steering assist mechanism isconnected to said return lines of said primary and auxiliary pumps.

11. In a vehicle hydraulic power steering system including a primarypump and an auxiliary pump and a steering assist mechanism connected toa transfer valve having a first position wherein said primary pump isconnected to said steering assist mechanism and having a second positionwherein said auxiliary pump is connected to said steering assistmechanism, the improvement which comprises, in combination, means forsensing mass flow rate of fluid from said primary pump and for producinga pressure differential corresponding thereto; and means for holdingsaid valve in said first position when said pressure differential isabove a predetermined level and for holding said valve in said secondposition when said pressure differential is below 7, said predeterminedlevel.

1. A vehicle power steering system which comprises: a primary pumphaving an output line and a return line; a secondary pump having anoutput line and a return line; means connected with said primary pumpoutput line for sensing mass flow rate of fluid therein; and valve meansconnected to, and for controlling fluid flow in, said primary andsecondary output and return lines, said valve means being connected tosaid sensing means and being responsive at a predetermined mass flowrate sensed thereby to alter the fluid flow circuits of said primary andsecondary pumps.
 2. A vehicle power steering system in accordance withclaim 1, wherein said means for sensing mass flow rate of fluidcomprises an orifice connected with said primary pump output line.
 3. Avehicle power steering system in accordance with claim 2, wherein saidmeans for sensing mass flow rate of fluid further includes one-way valvemeans connected in series with said orifice.
 4. A vehicle power steeringsystem, which comprises: a primary pump having an output line and areturn line; a secondary pump having an output line and a return line;means connected with said primary pump output line for sensing mass flowrate of fluid therein; a steering assist mechanism; and transfer valvemeans connected to, and for controlling fluid flow in, said primary andsecondary output and return lines, said valve means having a firstposition in which said primary pump output and return lines areconnected to said steering assist mechanism and wherein said secondarypump output and return lines are connected together by said valve means,and said valve means having a second position wherein said secondarypump output and return lines are connected to said steering assistmechanism for fluid flow therethrough.
 5. A vehicle power steeringsystem in accordance with claim 4, wherein said means for sensing massflow rate of fluid comprises fluid flow restrictive means for producinga fluid pressure differential.
 6. A vehicle power steering system inaccordance with claim 4, wherein said means for sensing mass flow rateof fluid comprises an orifice connected in series with one-way valvemeans to produce a pressure differential in the fluid output from saidprimary pump when said valve means are in said first position.
 7. Avehicle power steering system in accordance with claim 6, wherein saidtransfer valve means further include means responsive to a reductionbelow a predetermined level of the pressure differential across saidorifice and one-way valve means to shift said transfer valve means fromits first position to its second position.
 8. A vehicle power steeringsystem in accordance with claim 7, wherein said transfer valve means hasa third unstable intermediate position wherein the output from saidsteering assist mechanism is connected to said return lines of saidprimary and secondary pumps.
 9. A vehicle hydraulic power steeringsystem, which comprises: a primary hydraulic fluid pump having an outputline and a return line; an auxiliary hydraulic fluid pump having anoutput line and a return line; a fluid-controlled open-center-valvesteering assist mechanism having input and output lines connectedthereto; a transfer valve connected to the inpuT and output lines ofsaid steering assist mechanism, to the ouput line of said auxiliarypump, and to the return lines of said primary and auxiliary pumps, saidtransfer valve having a first position wherein said output line of saidsteering assist mechanism is connected to said return line of saidprimary pump and wherein said output line of said auxiliary pump isconnected to said return line of said auxiliary pump, and said transfervalve having a second position wherein said output line of saidauxiliary pump is connected to said input line of said steering assistmechanism and wherein said output line of said steering assist mechanismis connected to said return line of said auxiliary pump, said transfervalve further including first pilot means connected to said output lineof said primary pump for urging said transfer valve to its firstposition and second pilot means connected to said input line of saidsteering assist mechanism for urging said transfer valve to its secondposition; and orifice and one-way valve means for sensing mass flow rateof fluid flowing therethrough, said orifice and one-way valve meansbeing connected between said output line of said primary pump and saidinput line of said steering assist mechanism.
 10. A vehicle hydraulicpower steering system in accordance with claim 9, wherein said transfervalve has a third unstable intermediate position wherein said outputline of said steering assist mechanism is connected to said return linesof said primary and auxiliary pumps.
 11. In a vehicle hydraulic powersteering system including a primary pump and an auxiliary pump and asteering assist mechanism connected to a transfer valve having a firstposition wherein said primary pump is connected to said steering assistmechanism and having a second position wherein said auxiliary pump isconnected to said steering assist mechanism, the improvement whichcomprises, in combination, means for sensing mass flow rate of fluidfrom said primary pump and for producing a pressure differentialcorresponding thereto; and means for holding said valve in said firstposition when said pressure differential is above a predetermined leveland for holding said valve in said second position when said pressuredifferential is below said predetermined level.